Encoding scheme according to a moving picture expert group (MPEG) phase using a discrete cosine transform (DCT) has been known as a compressive encoding scheme of image signal. DCT process is done such that the discrete cosine transform is executed on pixels within each block to obtain coefficient data and then, the coefficient data thus obtained by the discrete cosine transform is re-quantized and the re-quantized coefficient data is encoded in variable length. With respect to such the variable length encoding, entropy coding such as Huffman coding has been often used. Orthogonal transform coding is executed on image data so that it is divided into many items of frequency data from low frequency to high frequency.
It has such an characteristic that a high quality image can be maintained and also a high efficient compression can be implemented, according to the way that, when re-quantizing the divided frequency data, low-frequency-component data, which is more significant, is fine quantized with taking eyesight of human being into consideration while high-frequency-component data, which is less significant, is coarsely quantized with taking eyesight of human being into consideration.
In conventional DCT decoding, the quantized data for each frequency component is transformed to its central value and it performs inverse DCT (IDCT) on those components, thereby acquiring reproduced data. A quantization step width in encoding is used when transforming it to the central value.
As described in the above, the encoding scheme according to MPEG using DCT has such a characteristic that a high-quality image can be maintained and also a high efficient compression can be implemented by encoding with taking eyesight of human being into consideration.
However, the DCT encoding is an encoding process based on a unit of block and thus, it exhibits that, when the compression rate is higher, a block-like noise, a known block noise (block distortion) may occur. A blotchy noise, a known mosquito noise, due to a coarse quantization on the high-frequency component also occurs in a part of image such as an edge having sharp luminance transition.
Such the encoding noises (encoding distortion) may occur in not only the encoding scheme according to MPEG but also another encoding scheme.